Recently, small-sized, cordless electronic appliances such as pocket telephones are being increasingly popularized. In addition, further development and popularization of electric cars is desired in view of environmental problems and energy problems. With these, secondary batteries having a high energy density are required.
Heretofore as good secondary batteries, known are nickel-cadmium batteries, nickel-hydrogen batteries, and lead-acid batteries. However, with the energy density these heavy batteries are not promising candidates.
On the contrary, since lithium secondary batteries comprise a light and easily-handlable lithium metal, their energy density is high, and therefore, they are expected to be applicable to high-performance pocket telephones and electric cars.
However, in lithium secondary batteries where a lithium metal is used in the negative electrode, the lithium metal is deposited as dendrite or powdery active metal on the surface of the negative electrode during charging a battery. In this case, the dendrite of a lithium metal often penetrates through a separator, and sometimes short-circuit the battery by electrically contacting with a positive electrode, or even reacts with the electrolytic solution, resulting in lowering the charge/discharge capacity of the lithium secondary batteries.
In order to evade these troubles, graphite is used as the negative electrode active material for lithium secondary batteries, so that graphite capable of being intercalated with lithium ions during charging could prevent the deposition of such dendritic lithium and the reaction of lithium with electrolytic solutions.
However, graphite is still problematic in that it has poor stickiness to a current collector through which generated electricity is taken out, therefore causing the deterioration of electrodes.
Japanese Patent Application Laid-Open No. 6-84515 discloses a negative electrode comprising graphite and, as a binder, from 40 to 60% by weight of coke, said coke being added to graphite in order to improve the stickiness of graphite to collectors.
Though having improved its stickiness, the electrode disclosed therein is still problematic in that it is often insufficiently charged due to voltage drop during high-rate charging operation.
On the other hand, coke itself can be used as a negative electrode, as disclosed in Japanese Patent Application Laid-Open No. 1-221859. Coke is advantageous in not only having much absorption capacity of lithium but also being inexpensive.
Coke as referred to in said laid-open specification is a gray-black porous solid obtained through high-temperature dry distillation (at 1200 to 1400.degree. C.) of oil or coal.
Japanese Patent Application Laid-Open No. 8-102324 discloses the powdered coke having heated at from 400.degree. C. to 700.degree. C. is calcinated in an inert gas atmosphere at from 900.degree. C. to 1500.degree. C., and applied to a negative electrode material.
However, the negative electrodes comprising any of such cokes are still problematic in the following points.
Since the negative electrodes comprising any of such cokes have poor electroconductivity and since the coke crystals constituting said cokes are large, the amount of lithium ions capable of being actually absorbed by the negative electrodes is still smaller than the theoretical capacity of graphite. Therefore, when said cokes are used, it is impossible to obtain lithium secondary batteries having a large charge/discharge capacity.
The theoretical capacity as referred to hereinabove is the capacity of a secondary battery in which it is presumed that the negative electrode active material is made of graphite that is the ultimate structure of carbon. In this, therefore, lithium ions are absorbed by the negative electrode active material through intercalation (in the first stage) of said lithium ions into the layers constituting the layer structure of said graphite. In this case, the graphite shall absorb one lithium ions per six carbon atoms. Accordingly, the theoretical capacity (that is, the charge/discharge capacity per gram of the negative electrode active material) is to be 372 mAhg.sup.-1.
On the other hand, (1) Japanese Patent Application Laid-Open No. 7-192724 discloses the use of a combination of a non-graphite carbon material and graphite; (2) Japanese Patent Application Laid-Open No. 7-147158 discloses the use of a graphite material containing from 1 to 30% by weight of pseudo-graphitic carbon black; and (3) Japanese Patent Application Laid-Open No. 7-326343 discloses the use of a carbon material combining high-crystalline graphite with low-crystalline carbon by melting.
It is known that these are effective in some degree for improving the capacity of negative electrodes for lithium secondary batteries.
However, the carbon material in (3) is still defined as crystallized carbon material rather than a mixture of graphite and coke; the amount of graphite in the material in (2) is larger than that inducing percolation transition; and the amount of graphite in the combination in (1) is not specifically defined to fall within the range that may induce percolation transition, and the amount of graphite and that of coke in the combination in (1) are not optimized. Thus, all these techniques disclosed are not still satisfactory in the point of increasing the capacity of negative electrodes for lithium secondary batteries.
The theoretical capacity as referred to hereinabove is the capacity of a secondary battery in which it is presumed that the negative electrode active material is made of graphite that is the ultimate structure of carbon. In this, therefore, lithium ions are absorbed by the negative electrode active material through intercalation (in the first stage) of said lithium ions into the layers constituting the layer structure of said graphite. In this case, the graphite shall absorb one lithium ions per six carbon atoms. Accordingly, the theoretical capacity (that is, the charge/discharge capacity per gram of the negative electrode active material) is to be 372 mAhg.sup.-1.
In consideration of these problems, the present invention is to provide a negative electrode material for lithium secondary batteries, with which it is possible to obtain lithium secondary batteries having a large discharge capacity.